Sulphide ooids from the proterozoic Siyeh Formation of Alberta,Canada

Sulphide ooids from 0.5 to 4 mm in diameter occur over a large area in a thin arenite bed near the base of the Proterozoic Siyeh Formation in southwestern Alberta. Examination by optical microscope, scanning electron microscope, and electron microprobe, reveals that the ooids are composed of concentric layers, a few micrometers to a few tens of micrometers thick, of pyrite and chalcopyrite. Covellite is also present in several ooids. Copper sulphides are more abundant towards the centre of the ooids than in the outer shells where pyrite is predominant. A slightly eccentric nucleus of chalcopyrite and/or titanium oxide occurs in several ooids. It is speculated that the ooids grew during early diagenesis of the arenite, and that the source of metals were black argillaceous sediments above and below the arenite. The nucleation of sulphides around detrital grains of chalcopyrite and titanium oxide has an important bearing on metallogenetic models of stratiform copper deposits.